So-called shape memory polymers or SMPs, which undergo a change in shape from a temporary shape into a permanent shape corresponding to a previous programming on induction by a suitable stimulus, are known in the prior art. This shape memory effect is most often stimulated thermally, i.e., the restoring effect driven by entropy elasticity occurs on heating the polymer material to a temperature above the defined transition temperature. Shape memory polymers are usually polymer networks in which chemical (covalent) crosslinking sites or physical (noncovalent) crosslinking sites determine the permanent shape. The programming takes place by deforming the polymer material to a temperature above the transition temperature of a “switching segment” and then cooling the polymer to a temperature below this temperature while maintaining the deformation forces to imprint the temporary shape. Renewed heating to a temperature above the transition temperature leads to a phase transition and restoration of the original permanent shape.
In addition, polymer networks having two switching segments with different transition temperatures have also recently become known.
For example, EP 1 362 879 A describes shape memory polymers (interpenetrating networks IPNs in this case) comprising a covalently crosslinked polymer component, in particular based on caprolactone, lactide, glycolide or p-dioxanone units, and a noncovalently crosslinked polyester urethane component. The polymer is able to store two temporary shapes, for which transition temperatures of 50° C. and 90° C. have been reported.
Liu et al. (Macrmol. Rap. Comm. 26, 2005, 649ff) describes an SMP (semi-interpenetrating network SIPN) comprising polymethyl methacrylate units (PMMA) and polyethylene glycol units (PEG) and also having two transition temperatures (at 40° C. and 86° C.). However, the programming method described there allows only a temporary shape to be stored.
One disadvantage of the known shape memory polymers for many applications is that their switching temperatures are relatively close together, which thus requires accurate setting of the temperature when heating between two transition temperatures. Furthermore, the relatively low transition temperatures may be problematical for certain applications, namely if high application-related temperatures occur without restoration of the permanent shape being desired.